PROJECT SUMMARY Respiratory syncytial virus (RSV) is the leading cause of severe lower respiratory tract disease in young children worldwide, and is also a major cause of morbidity in the elderly and immunocompromised populations. No approved RSV vaccine exists. Our goal is to utilize a structure-guided design approach to rationally engineer RSV G protein immunogens that induce safe and protective immunity against RSV. RSV G protein is one of two major immunogenic proteins on the RSV surface and has key roles in virus attachment to airway epithelial cells and virus modulation of innate immune defenses. RSV G protein is the target of broadly neutralizing, protective antibodies, however its development as a vaccine immunogen has been hampered by safety concerns with its immune-modulating activity on chemokine receptor CX3CR1+ cells and by a paucity of structural information on its epitopes. In this proposal, we will test our central hypothesis that an engineered RSV G immunogen that cannot activate CX3CR1+ cells but does display protective conformational epitopes will elicit safe and effective RSV immunity. We will use an integrated, multidisciplinary approach with experts in RSV G protein structure and engineering (DuBois) and RSV virology and immunology (Tripp) to pursue four specific aims: (1) Use structural studies to define both the conserved RSV G protein epitopes recognized by protective antibodies and the binding site for the chemokine receptor CX3CR1, (2) Use structure-guided design to engineer RSV G protein immunogens, (3) Use a multi-parameter immunology approach to evaluate 3 engineered RSV G protein immunogens for improved safety and immunogenicity, and (4) Evaluate 2-3 engineered RSV G protein immunogens for safety and ability to prevent RSV infection and disease in mice. The proposed research will generate a single engineered RSV G vaccine immunogen with demonstrated safety and efficacy in preventing RSV infection and disease in vivo.